There are two general modes in which the frequency of a wave can be measured. The number of cycles of the wave that occur during a given period of time can be counted or the number of cycles from a reference oscillator that occur during a given number of cycles of the wave can be counted. The first mode is called the frequency mode and the second is called the period mode. In the period mode the frequency of the wave is determined by dividing a given number of cycles of the wave by the number of cycles of an oscillator that occur during said given number of cycles and multiplying by the frequency of the oscillator. The resolution of a frequency measurement depends on the number of cycles that are counted. If there is sufficient time for making a measurement, any desired resolution can be obtained with either mode but if the measurement must be made in a short time, the frequency mode gives better solution for frequencies above a given value and the period mode gives better resolution for frequencies below that value. Furthermore, if the maximum frequency to be measured is greater than the highest frequency that can be handled by the counter circuitry, a frequency divider is connected ahead of the counter circuitry, but when the frequency of the wave equals this highest frequency, the frequency divider should be bypassed in order to obtain better resolution. When most instruments are operated in the period mode, it is possible to select different numbers of cycles of the wave being measured during which the cycles of the oscillator are counted. If only one number of cycles is available, the resolution of a measurement increases as the frequency of the wave being measured becomes lower, but the time required for the measurement also increases. By way of example, if cycles of a 10 MHz oscillator are counted during 1000 cycles of the wave, it would take 0.01 seconds to measure the frequency of a wave having a frequency of 100K and 100K counts would occur, but if the wave being measured has a frequency of 100 cycles, it would take ten seconds to make the measurement. Furthermore, the number of oscillator cycles occurring during this time would be 100M and might cause the counter to overflow. Thus it is advantageous to be able to reduce the number of cycles of a wave during which the cycles from the oscillator are counted.
Whereas placing the divider in or out of the circuit, the selection of the frequency mode or period mode of operation and the selection of the number of cycles of the wave being measured during which the cycles at the output of the oscillator are counted in a period mode of operation may be done manually, it would take considerable experimentation to obtain the right combination of these factors unless the frequency of the wave is approximately known, in which event the combination could be set in advance. In many applications there is not enough time to carry out the experimentation.